Vacuum may be used to operate or to assist in the operation of various devices of a vehicle. For example, vacuum may be used to assist a driver applying vehicle brakes, turbocharger operation, fuel vapor purging, heating and ventilation system actuation, and driveline component actuation. Vacuum may be obtained sometimes from an engine intake manifold in normally aspirated engines because the intake manifold pressure is often at a pressure lower than atmospheric pressure. However, in boosted engines where intake manifold pressures are often at pressures greater than atmospheric pressure, intake manifold vacuum may replaced or augmented with vacuum from an ejector. By passing pressurized air though the ejector, a low pressure region may be created within the ejector so that air can be drawn from a vacuum reservoir to the ejector, thereby reducing pressure within the vacuum reservoir. Nevertheless, use of ejector systems may not be desirable during certain engine operating conditions where there may be insufficient air flow from the compressor to provide a desired engine torque.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for providing vacuum, comprising: supplying air from a compressor to engine cylinders, the compressor having an air output capacity; directing air flow from the compressor to an ejector supplying vacuum to a vacuum reservoir when the compressor has capacity to supply more air than a desired cylinder air amount, otherwise inhibiting air flow from the compressor to the ejector.
By adjusting when air is supplied from a compressor to an ejector, it may be possible to supply a desired amount of vacuum to a vacuum system while at the same time providing an engine air amount that in near the desired engine air amount. For example, during a throttle application by a driver, air flow to an ejector can be inhibited. Shortly after the turbine spins up to speed, air flow to the ejector may be reactivated. Thus, generation of vacuum via the ejector may only be inhibited for a short period of time. Consequently, vacuum may be generated for a substantial amount of time even during vehicle acceleration when intake manifold pressure may be high. Further, if an engine torque demand exceeds a threshold level, air flow to the ejector may be inhibited to provide additional engine torque. Thus, an objective of providing engine torque may be granted priority over producing vacuum during some engine operating conditions. Additional vacuum may be provided via the ejector when the output capacity of the turbocharger compressor exceeds the desired engine air charge. In this way, it may be possible to provide vacuum to a vacuum system while providing a desired amount of torque.
The present description may provide several advantages. For example, the approach may improve engine fuel economy by providing vacuum based on vacuum use rather than simply supplying a high level of vacuum. Further, the approach can prioritize vacuum use and vacuum generation according to operating conditions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.